This invention relates to a flexible magnetic recording medium for use in perpendicular magnetic recording and, more particularly, to a flexible magnetic recording medium which comprises an underlying film having in-plane magnetic anisotropy under a surface film having perpendicular magnetic anisotropy.
A conventional flexible magnetic recording medium of the type described comprises a substrate, such as a polyester film, which has a principal surface. A first film is formed on the principal surface as an underlying film. A second film is formed on the first film as a surface film. The second film has magnetic anisotropy perpendicular to the principal surface, that is, an axes of easy magnetization perpendicular to the second film. For brevity of description such magnetic anisotropy will be called "perpendicular magnetic anisotropy". The second film is magnetizable perpendicular to the principal surface.
On the other hand, the first film is of an alloy having high magnetic permeability. The first film has magnetic anisotropy parallel to the principal surface, that is, both axes of easy magnetization and axes of hard magnetization are parallel to the principal surface. For brevity of description, such magnetic anisotropy will be called "in-plane magnetic anisotropy". As is well known in the art, the first film is operable as a part of a magnetic head on perpendicularly recording a digital signal into the second film and on reading the digital signal out of the second film.
In the flexible magnetic recording medium, the first film is of a composition of nickel, iron, and molybdenum, known as "Permalloy". Otherwise, the first film is of another composition of nickel, iron, molybdenum, and copper known also as "Permalloy". In general, the first film of the Permalloy having the above-mentioned composition is continuously formed on the substrate while the substrate is fed lengthwise through a sputtering device. The first film formed on the substrate in the above-mentioned manner has the in-plane magnetic anisotropy with axes of hard magnetization directed lengthwise of the substrate and with axes of easy magnetization directed widthwise of the substrate. The conventional first film has saturation magnetization which is equal to or greater than 550 e.m.u./cc.
A flexible disk is cut from a wide flexible magnetic medium of the type described above. Such a flexible disk is well known in the art as a "floppy disk". Let a signal having a constant voltage amplitude be recorded into the flexible disk. As will later be described in conjunction with a few figures of the accompanying drawing, a voltage amplitude of a reproduced signal read out of the disk varies during one complete revolution of the disk so that a maximum voltage amplitude and a minimum voltage amplitude of the reproduced signal appear twice each. This is because the in-plane magnetic anisotropy of the first film is too large because there is a too large saturation magnetization of the first film. Thus, the first film has so large an in-plane magnetic anisotropy that it is not possible to accurately record digital information by the use of the flexible disk.